Fuel cells, which generate electric current by the electrochemical combination of hydrogen and oxygen, are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by an electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a Solid-Oxide Fuel Cell (SOFC). SOFC systems derive electrical power through a high-efficiency conversion process from a variety of fuels including natural gas, liquefied petroleum gas, ethanol, and other hydrocarbon and non-hydrocarbon fuels. Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode.
Each O2 molecule is split and reduced to two O−2 anions catalytically by the cathode. The oxygen anions transport through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and cathode are connected externally through a load to complete the circuit whereby four electrons are transferred from the anode to the cathode.
When hydrogen as a feed stock for the fuel cell is derived by “reforming” hydrocarbons such as gasoline in the presence of limited oxygen, the reformate gas includes CO which is converted to CO2 at the anode via an oxidation process similar to that performed on the hydrogen. A single fuel cell is capable of generating a relatively small amount of voltage and wattage and, therefore, in practice it is known to stack a plurality of fuel cells together in electrical series.
Reformed gasoline is a commonly used feed stock in automotive fuel cell applications. Reformate gas is typically the effluent from a catalytic liquid or gaseous hydrogen oxidizing reformer and is often referred to as “fuel gas” or “reformate”. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the hydrocarbon fuel, resulting ultimately in water and carbon dioxide. Both reactions are preferably carried out at relatively high temperatures, for example, in the range of about 700° C. to about 900° C.
Since optimum fuel consumption and electrical generation, and therefore optimum efficiency of a SOFC stack, are reached at relatively high operating temperatures, a cooling process needs to be performed during normal system shutdowns. The normal system shutdown is a period that occurs, for example, prior to an extended duration of nonuse. Typically, the SOFC stack is cooled with air utilizing the cathode airflow. Due to the relative high operating temperature of the SOFC stack, typically about 750° C. and higher, and the chemical composition of the anodes, purging the entire SOFC stack with cathode air for cooling prior to extended periods of nonuse results in a degrading and fatiguing oxidation of the anodes since oxygen is allowed in the cavities adjacent to the anodes. The anode side of the fuel cell is, in part, nickel. At temperatures above about 400° C., and in the presence of free oxygen, nickel oxide is formed, which may lead to deterioration of the SOFC stack and which over time may cause failure of the SOFC stack. Therefore, it is harmful to the SOFC stack when oxygen is allowed in the cavities adjacent to the plurality of anodes.
The currently accepted method for preventing the oxidation of the anodes in a laboratory environment is a process in which the cavities in the anode side of the SOFC stack are continuously purged with a fluid containing no free oxygen during a normal cooling process. For example, a blend of bottled reducing gas may be flowed through the anode side of the SOFC stack while the system cools from its operating temperature to a temperature below about 400° C. where harmful oxidation of the anodes ceases. Though a purge is required, such continuous purging process requires more pressurized reducing gas than can be stored on an onboard system, such as what might be needed in a non-stationary motor vehicle.
A typical SOFC system requires usually between four to eight hours to cool from its operating temperature to a temperature that will not harm the anode side of the stack. During this time, a continuous purging process will exhaust more reducing gas than a volume of storage bottles, equal to the size of the entire system, can hold. While purging the anode side of the stack during a normal cooling process with a reducing fluid produced by the SOFC system itself, such as utilization of exhaust gas where oxygen has been reduced below ignition concentration, has been proposed, this is not a solution for emergency system shutdowns when an unexpected event requires an immediate shutdown of the entire SOFC system including the use of external power. In such emergency situation, the SOFC system does not have time to execute its normal shutdown process and the anode components of the stack cannot be protected from detrimental oxidation. No protecting mechanisms for the anode side of the stack are currently known for such emergency situation.
What is needed in the art is a cooling apparatus and strategy that prevents detrimental oxidation of the anode side of the stack from occurring in the event that a sudden and complete emergency shutdown of the SOFC system is necessary.
It is a principal object of the present invention to provide an apparatus that enables a SOFC system to safely cool down during an emergency system shutdown, while the anodes of the stack are protected from oxidation by a reducing environment.
It is a further object of the invention to provide an apparatus that requires only a fraction of space required by prior art gas bottles to provide the desired oxygen-free environment.